Self-association of model transmembrane alpha-helices is modulated by lipid structure

We have developed a fluorescence quenching method using peptides containing 3,5-dibromotryrosine to measure oligomerization of model transmembrane a-h elices in lipid bilayers. Peptides of the type Ac-LysLysGlyLeu(m)XLeu(n)Lys LysAla-amide where X is tryptophan or 3,5-dibromotyrosine were found to for m heterodimers in bilayers of phosphatidylcholine in the liquid-crystalline phase. The free energy of dimer formation changed little with increasing n umber of Leu residues from 16 to 22 but increased with increasing phospholi pid fatty acyl chain length, with a slope of about 0.5 kJ mol(-1) per fatty acyl chain carbon. Peptides were excluded from lipid in the gel phase, res ulting in increased levels of oligomerization. Addition of cholesterol to f orm the liquid-ordered state led to increased dimerization but without phas e separation. The presence of phosphatidylethanolamine had little effect on dimerization.